1. Field of the Invention
The present invention relates to a zoom lens apparatus used in a camera such as TV camera and video camera.
2. Description of the Related Art
A rear focus type zoom lens with a focus lens unit (rear focus lens unit) placed closer to the image plane than a variator lens unit has an advantage in reducing size and weight of the focus lens unit, and is therefore widely used for an autofocus type zoom lens.
On the other hand, a front focus system with a focus lens unit (front focus lens unit) placed closer to an object than a variator lens unit keeps the same amount of movement of the focus lens unit even if magnification is changed and has an advantage for the variator lens unit, and is therefore widely used for a zoom lens apparatus for broadcasting and business which attach importance to manual operations.
In view of the above described positions, a zoom lens which uses a focus lens unit closer to the image plane side than a variator lens unit for autofocusing and uses a focus lens unit closer to an object than a variator lens unit for manual focusing is disclosed in Japanese Utility Model Publication No. S62(1987)-43286, Japanese Patent Publication No. H5(1993)-6163, Japanese Patent No. 2505835, Japanese Patent No. 2505840 and Japanese Patent No. 2561637.
For example, broadcast or professional video zoom lenses carry out faster and more accurate focusing operations or intentional focusing operations for image effects compared to consumer ones. However, these focusing operations may not be sufficiently supported by autofocusing alone, and so requires auxiliary manual focusing operations for autofocusing.
More specifically,    1. High-speed manual focusing operation: For example, when an out-of-focus amount is large and it is difficult to make a focusing determination or when manual focusing is carried out up to a position close to an in-focus position    2. High-accuracy manual focusing operation: When fine focusing is carried out through manual focusing, for example, when the nose of a person as an object is focused, the focus is changed to the eye    3. Intentional manual focusing operation: For example, when blurring is applied intentionally, when an object near the perimeter of the screen is focused or when the focus is switched among a plurality of objects at different distances at a fixed speed.
However, the focus lens unit with the zoom lenses disclosed in the above described publications cannot operate fine focusing because the respective front focus lens units are fixed during autofocusing.
Furthermore, it is also possible to use a rear focus lens unit for fine manual focusing during autofocusing, but it requires an operation mechanism different from the operation mechanism of the front focus lens unit such as an electronic ring, which makes the configuration complicated and produces an uncomfortable feeling during operation to users such as TV camera operators who are used to focusing operations using the front focus lens unit.
Furthermore, when a front focus lens unit is allowed to be moved without limitation during autofocusing, an in-focus range by autofocusing may be restricted by the position of the front focus lens unit, which will prevent the focusing from being achieved in the vicinity of infinity or the minimum object distance.